Liquid crystal displays continue to improve in cost and performance, becoming a preferred display type for many computer, instrumentation, and entertainment applications. While LCDs are advantaged for minimal thickness, low weight, and reasonably good color characteristics, there are some significant drawbacks with this display technology. Among the major drawbacks of LCDs are relatively low brightness and disappointing contrast.
The transmissive LCD used in conventional laptop computer displays is a type of backlit display, having a light providing surface positioned behind the LCD for directing light outwards, towards the LCD. The challenge of providing a suitable backlight apparatus that is both compact and low cost has been addressed following one of two basic approaches. In the first approach, a light-providing surface is used to provide a highly scattered, essentially Lambertian light distribution, having an essentially constant luminance over a broad range of angles. Following this first approach, with the goal of increasing on-axis and near-axis luminance, a number of brightness enhancement films have been proposed for redirecting a portion of this light having Lambertian distribution in order to provide a more collimated illumination. Among proposed solutions for brightness enhancement films are those described in U.S. Pat. No. 5,592,332 (Nishio et al.); U.S. Pat. No. 6,111,696 (Allen et al); U.S. Pat. No. 6,280,063 (Fong et al.); and U.S. Pat. No. 5,629,784 (Abileah et al.), for example. Solutions such as the brightness enhancement film (BEF) described in the patents cited above provide some measure of increased brightness over wide viewing angles. However, overall contrast, even with a BEF, remains relatively poor.
A second approach to providing backlight illumination employs a light guiding plate (LGP) that accepts incident light from a lamp or other light source disposed at the side and guides this light internally using Total Internal Reflection (TIR) so that emitted light has a restricted range of angles. The output light from the LGP is typically at a fairly steep angle with respect to normal, such as 80 degrees or more. With this second approach, a turning film is then used to redirect the emitted light output from the LGP toward normal. Directional turning films, broadly termed light redirecting films, such as that provided with the HSOT (Highly Scattering Optical Transmission) light guide panel available from Clarex, Inc., Baldwin, N.Y., provide an improved solution for providing a uniform backlight of this type, without the need for diffusion films or for dot printing in manufacture. HSOT light guide panels and other types of directional turning films use arrays of prism structures, in various combinations, to redirect light from a light guiding plate toward normal, relative to the two-dimensional surface. As one example, U.S. Pat. No. 6,746,130 (Ohkawa) describes a light control sheet that acts as a turning film for LGP illumination.
Improving upon this second approach, U.S. Pat. No. 6,421,103 entitled “Liquid-Crystal Display Apparatus Including a Backlight Section Using Collimating Plate” to Yamaguchi and U.S. Pat. No. 6,327,091 entitled “Collimating Plate and Backlight System” to Agano both disclose providing collimated illumination from the backlight to the LCD. It has been shown that collimated light generally improves the contrast ratio of the LCD and, when used in conjunction with a diffuser for the modulated light, may effectively increase the usable viewing angle. Referring to FIG. 1A, an LC display 10 has a backlight unit 12 that provides collimated illumination, at a normal angle, to an LC device 14. LC device 14 has a liquid crystal material sandwiched between transparent substrates. A diffuser 16 then diffuses the modulated light from LC device 14, expanding the viewing angle as indicated in FIG. 1A. Referring to FIG. 1B, there is shown a cone of illumination 20 about a central ray 22 provided from backlight unit 12. Central ray 22, sometimes alternately termed a chief or principal ray, is normal with respect to the surface of LC device 14. The above approaches provide some amount of increased contrast when using LGP illumination. However, there is still room for improvement. Moreover, it is not clear whether or not further contrast gains achieved using this approach might be offset by loss of viewing angle.
An alternate strategy for optimizing contrast is directed toward optimizing performance of the LC device itself. Referring to FIGS. 2A, 2B, and 2C, there are shown conventional contrast plots obtained for a Twisted Nematic (TN) LC device using the EZCONTRAST Conoscope for measuring luminance, contrast, and color, from Eldim Corporation, Saint Clair, France. FIG. 2A is obtained for a TN LC device at a low voltage (3.6V nominal). FIG. 2B shows contrast for a higher voltage level (4.0V nominal). FIG. 2C shows contrast for an even higher voltage level (5.0V nominal). A maximum contrast area 24 denotes the highest contrast characteristic, which moves toward normal as voltage increases. (In the graphical representation of FIGS. 2A-2C, measurements for the horizontal and vertical axes are as labeled in FIG. 2A.) As the progression of FIGS. 2A, 2B, and 2C indicates, there is measurable contrast improvement for TN LCDs with increased drive voltage. The fact that the maximum contrast does not necessarily occur at the normal direction also holds for other LC types, including Vertically Aligned (VA) LCDs and Optically Compensated Birefringence type (OCB) LCDs, among others.
Since the progression of FIGS. 2A-2C shows improved contrast at higher voltages, the approach for increasing the contrast ratio that suggests itself is simply to employ higher drive voltages. While this is straightforward, such an approach has undesirable ramifications. In order to handle the additional power requirement for improved contrast at normal angles, it is necessary to fabricate the Thin-Film Transistor (TFT) devices that drive each LCD pixel using thicker electrode materials and larger components. This adds cost to LC device fabrication and imposes size constraints that limit display resolution. Thus, increased drive voltage has proved to be of limited value for providing higher device contrast.
While methods described above have provided some incremental improvements in display contrast, further contrast ratio enhancement is necessary in order to use LCDs more effectively for images having subtle detail, such as those used for medical diagnosis. Thus, there is a need for apparatus and methods that enhance the contrast of LC display devices without increasing device cost, complexity, or size.